1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device utilizing SH-type SAWs, whose main displacement is in the direction perpendicular to the direction of the SAW propagation, such as a BGS wave (Bleustein-Gulyaev-Shimizu Wave).
2. Description of the Prior Art
In a SAW device, the adhesion of an elastic material on a SAW propagation path and an interdigital transducer (IDT) has been conventionally put under taboo. The reason for this is that a SAW is absorbed by the elastic material so that an output of the SAW is damped. Consequently, in constructing the SAW as a component, a cavity for not preventing the propagation of the SAW has been provided on the SAW propagation path and the IDT. As a result, a package structure of the SAW device has been complicated and high in cost.
On the other hand, Japanese Patent Laid-Open Gazette No. 108551/1979 discloses a structure in which the above described cavity is omitted in a SAW filter utilizing a BGS wave. More specifically, the prior art discloses a structure in which insulating rubber in the form of a viscous material at room temperature and in an atmosphere and cured by the heating or moisture absorbing is arranged on an IDT and a SAW propagation path, thereby to omit the cavity. It is described that an output of the BGS SAW filter is not damped even if the above described insulating rubber is arranged on the IDT and the SAW propagation path. As the above described insulating rubber in the form of a viscous material at room temperature and in an atmosphere and cured by the heating or moisture absorbing, only silicone rubber cured by moisture absorption is specifically shown.
However, the SAW resonator utilizing a BGS wave has the disadvantage in that if the IDT and the SAW propagation path are coated with the silicone rubber, the impedance at a resonant frequency and the propagation loss are significantly increased, so that the resonance characteristics and filter characteristics are degenerated.